Thank you for your replies.
I followed the messages below from elcico2001 and "webmaster for Kracked Press Productions" about installing Sanskrit 2003 and changing preferences.
Trisquel 4.5.1 is a little different from what elcico2001 wrote---instead of a "Disposition" tab, there is a "Layouts" tab. I went to the following places:
System
> Preferences
> Keyboard
> Layouts (tab)
> Add (button)
> By language (tab)
> Language (drop-down list)
> Hindi.
On the GNOME panel at the bottom of my desktop, there are two keyboard icons, one with an earth ("Keyboard preferences"), and one without ("IBus input method framework").
Right-clicking the icon without an earth gives a menu with radio buttons for "USA" and "India Hindi Bolnagri".
When I chose "India Hindi Bolnagri", I was able to type Devanaagarii (Hindi, Sanskrit) in Writer.
@soumalya ray,
@Tom Davies:
Trisquel 4.5.1 has System > Preferences, but no Language.
Winston